The present invention relates to a method for manufacturing a semiconductor device and, more particularly, to a method for manufacturing an integrated circuit including a MIS field-effect transistor.
2. Description of the Prior Art
In the manufacture of integrated circuits, word lines for transmitting address data to memory cells are often made of the same material as that of a gate electrode of a MISFET. For example, as shown in FIG. 1, a cell array 1 of an integrated circuit basically consists of cells 2, word lines 3 and bit lines 4. The word lines 3 are connected to a gate electrode of each transistor called a transfer transistor. In an integrated circuit having a semiconductor silicon substrate, polycrystalline silicon is used for the gate electrode. Accordingly, the word lines 3 are often also made of polycrystalline silicon. Furthermore, diffusion layers for sources or drains which have the opposite conductivity type compared to the semiconductor substrate may also serve as the bit lines 4, or as power supply lines or ground lines.
A short response time is of prime importance in integrated circuits. However, delay (RC delay) in data transmission due to wiring resistance has prevented such a short response time. That is, the polycrystalline silicon constituting the word lines is electrically a semiconductor and therefore has a relatively high resistance. Such a high resistance of polycrystalline silicon results in a delay in data transmission through the word lines and hence a delay in signal transmission of an integrated circuit. When diffusion layers for sources or drains are used as bit lines, power supply lines or ground lines, a similar problem is encountered, since the diffusion layers are also electrically semiconductors.
In view of this problem, one proposal is to use an aluminum gate electrode. However, since aluminum has a melting point which is as low as 660.degree. C., it cannot withstand annealing for formation of diffusion layers. Accordingly, this proposal is impractical.